Inkjet printing systems are generally of two types: drop-on-demand (DOD) printing systems and continuous inkjet (CIJ) printing systems. Inkjet printing is a standard method for printing a colored image onto a substrate wherein a stream of ink droplets is directed from a printing device to a surface of a suitable receiver element or substrate. The direction of the stream of droplets is controlled electronically causing the droplets to print the desired image or information on the substrate surface without requiring contract between the printing device and the surface to which the ink is applied. Objects comprising substrates to which inkjet printing is well suited include but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, identification cards, lottery tickets, bank cards, identification strips, labels, brochures, signage, and other well-known materials.
Drop-on-demand printing systems are widely used in home or consumer inkjet printers and slower consumer printers, both of which have been available for several decades. As the name implies, this type of inkjet printing uses a print head that ejects drops of ink only when signaled to do so by a digital controller.
CIJ printing systems generally comprise two main components, a fluid system (including an ink reservoir) and one or more print heads. Ink can be pumped through a supply line from the ink reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head(s). Stimulations can be applied to the print head(s) to cause those ink streams to form streams of uniformly sized and spaced drops that are deflected in a suitable manner, into printing or non-printing paths. Non-printing drops can be returned to the ink reservoir using a drop catcher and a return line. Thus, in contrast to DOD printing systems, CIJ printing systems involves use of a continuous stream of ink drops that are separated to discriminate between spaced printing drops and non-printing drops. This discrimination can be accomplished by electrostatically charging the drops and passing the charged drops through an electric field. Charged drops are deflected by a charge field and can be caught and returned to the reservoir of ink. Uncharged drops are printed on a substrate or receiver material. This discrimination can also be accomplished when the printhead digitally creates large and small drops. Small drops are deflected by an air current and returned to the reservoir of ink. Large drops, being less affected by the air current, are printed onto a substrate or receiver material. Some useful CIJ printing apparatus and printhead fabrication are described for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.) and U.S. Pat. No. 6,943,037 (Anagnostopoulos et al.).
Commercially available CIJ inks are mostly aqueous dye-based inks that exhibit a number of problems. In such dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based inkjet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness, poor smear resistance, and uncontrolled gloss.
To address many of these problems, pigment based inks have been developed and incorporated into commercial CIJ systems as noted in the patents cited above and various publications cited therein. In addition, U.S. Pat. No. 7,537,650 (Szajewski et al.) describes inkjet sets containing a colored aqueous ink (with cationic coloring agent or pigment) and a substantially colorless aqueous ink that contains an anionic polymer or oligomer. Such inks can be applied in substantially an overlaying manner. Both inks are generally applied to the same areas of a receiver element so that the mixing of cationic and anionic materials in the inks provide a more consistent single color density on a variety of substrates, thereby reducing intercolor bleed and non-uniformities.
In general, such pigment-based colored inks can comprise a wide variety of colored pigments that can be chosen depending upon the specific application and performance requirements for the printing system and desired printing results. For example, such pigments can include but are not limited to, carbon black or other black pigments, red pigments, green pigments, blue pigments, orange pigments, violet pigments, magenta pigments, yellow pigments, and cyan pigments. The printed images using such pigment-based inks are generally desired to have a visual density of at least 0.5.
Colorless or invisible aqueous inkjet printer ink compositions containing various fluorescing pigments, and optionally colored non-fluorescing pigments, are described for example in U.S. Pat. No. 8,349,211 (Cai et al.) and in U.S. Patent Application Publication 2014/0231674 (Cook).
A pigment is generally desirable in inkjet ink compositions to provide resistance to fading. However, pigments generally must be physically milled to produce particles of desired small size to provide sufficient colloidal stability to the particles, with or without a suitable dispersant.
Pigmented inkjet ink compositions may have limited durability after printing on receiver elements (“substrates”), especially under conditions where abrasive forces are applied to the printed images. Furthermore, the printed images on the receiver elements are susceptible to defects from immediately after printing to several minutes while the inkjet printer ink compositions are drying. The durability of the dried printed images is also subject to environmental factors such as temperature and humidity that, under certain circumstances, can degrade image durability.
To solve these various problems, pigmented aqueous inkjet ink compositions have been formulated with various polymers, dispersants, and other addenda to provide more durable images under printing physical abuse and environmental conditions. Such inkjet ink compositions are described in U.S. Patent Application Publication 2008/0207811 (Brust et al.) and U.S. Pat. No. 8,192,008 (Brust et al.). Polyurethanes and polyureas are described as additives in inkjet ink compositions in U.S. Patent Application Publications 2009/0169748 (House et al.), 2009/0169749 (Brust et al.), and 2012/0050380 (Falkner et al).
Although the noted aqueous inkjet ink compositions provide improvements in the durability of inkjet printed images, when the polyurethane has too high an acid number, the web rub durability of the resulting printed image is decreased.
Inkjet ink sets are described in U.S. Patent Application Publication 2008/0207811 (Brust et al.), which sets include pigment-based color inks and pigment-free colorless inks, and the individual inks contain an acidic polyurethane binder to improve high temperature stability. Similar pigment-based colorless inks are also described in U.S. Pat. No. 8,187,371 (Brust et al.), which inks contain a polycarbonate-type polyurethane and an acrylic polymer.
DOD colorless inkjet compositions and systems are described in WO 2014/021840 (Iu et al.) that also describes the application of the colorless composition to provide print quality, gloss, scratch resistance, and rub resistance. Such aqueous-based inks comprise various solvents and additives such as essential wax particles and polymeric binders to provide durability. The wax particles are present in an amount of from 0.3% and up to and including 6% of the total inkjet ink composition weight.
While the use of wax particles may improve durability of inkjet printed images from DOD imaging apparatus, the presence of wax particles creates problems for many CIJ imaging systems and apparatus in that dispersions of such particles can become unstable and aggregate under the high shear conditions of continuous ink jetting and recirculation. The larger aggregated wax particles interfere with the drop formation process, eventually leading to plugged filters, plugged nozzles and crooked or missing jets. So, there is a need to avoid the use of wax particles without sacrificing printed image durability.
While the art has described useful colorless pigment-based ink compositions, particularly for DOD printing applications, there continues to be need for improved aqueous colorless inkjet ink compositions that provide enhanced gloss and durability when ink jetted onto at least part of a color image that has for example been formed by a pigment-based colored inkjet composition. Moreover, there is a need to be able to control the glossiness and durability for different types of colored inkjet ink printed images for example, by adjusting the chemical composition of the aqueous colorless inkjet ink compositions.